However, there was no difference between the groups when it came to rating their own performance in the tests.

The drug users denied there were any disruptive effects to their memory function.

Dr Rodgers said the researchers used the Weschler Memory Scale (Revised).

She said this gauges memory performance by getting subjects to retell stories.

Short-term memory is assessed by the subject's ability to retell the story straight after they have heard it.

Their ability to retell it an hour later gives an impression of long-term memory performance.

The drug users had problems recalling the story they heard, Dr Rodgers said.

"In most instances it was forgetting a significant amounts of information, so it wasn't inaccuracy that we found, it was more the case that the amount that was recorded was much less."

Important to consider both drugs

Dr Rodgers said: "Given that cannabis is seen as a useful substance for ameliorating the midweek low associated with (weekend) Ecstasy use, it's important to test for memory impairment in both types of drug user."

She told BBC News Online there are number of possible explanations why drug users were unaware of their memory problems.

"One of them could be that the drug users have a meta-cognitive deficit, which means they're not very good at assessing their own cognitive performance," she said.

The researchers now plan more focussed studies to establish if this is the actual cause.

Caution over conclusions

However, the Health Education Authority urged caution in interpreting the study's results.

A spokesman said: "We would not dispute the findings but would urge caution because of the small sample size."

The authority was also concerned that there was no indication of the extent to which the subjects used drugs.

"We look forward to seeing the research peer reviewed by other psychologist," he said.

Brain damage link

Research published last year suggested that Ecstasy could cause brain damage.

It found that each tablet did damage equivalent to a hammer blow to the head.

The researchers said the damage could cause memory disturbance, depression, anxiety and psychiatric disorders.

This is because the nerves it affects are those which release serotonin, which carries messages between nerves.

Serotonin, otherwise known as 5-HT, is thought to play a role in regulating memory, mood, perception of pain, sleep, appetite and libido.